Abstract. We consider the Schroedinger equation for a class of two-level atoms
in a quasi-periodic external field for large coupling, i.e., for which
the energy difference between the unperturbed levels is sufficiently
small. We show that this equation has a solution in terms of a formal
power series, with coefficients which are quasi-periodical functions
of the time, in analogy to the Lindstedt-Poincare' series in classical
mechanics.